This invention relates generally to ultrasonic testing and more particularly to measurement methods in ultrasonic testing.
Ultrasonic surface waves are often employed to detect or characterize near surface residual stresses, material property changes, surface roughness, defects, etc. in a specimen. Typically an immersion ultrasonic technique is used, wherein one or more transducers are acoustically coupled to a specimen by a coupling medium, for example liquid water. Ultrasonic waves can then be transmitted from the transducer to the specimen and back through the coupling medium. One of the most important quantities measured in such a technique is the traveling time of a surface wave between two points on the surface of the test specimen. Using the prior art immersion ultrasonic technique, there is no practical method to accurately extract this true surface wave traveling time from the total traveling time of the ultrasonic wave going from the transmitting transducer to the receiving transducer. This total traveling time includes not only the true surface wave traveling time in the specimen, but also the traveling time of the wave through the coupling medium between the specimen and the transducers.
Accordingly, there is a need for a practical method of accurately measuring the true surface wave traveling time between two points on a specimen.
The above-mentioned need is met by the present invention, which provides a method and apparatus for measuring the surface travel time of a surface wave between first and second points on a surface. First and second spaced apart ultrasonic transducers are disposed with their longitudinal axes substantially perpendicular to a surface. The direct travel times for each transducer are determined, then a total travel time from the first transducer to the second transducer is measured. The direct travel times are subtracted from the total travel time to compute the actual travel time between the points on the surface.
The present invention and its advantages over the prior art will become apparent upon reading the following detailed description and the appended claims with reference to the accompanying drawings.